Australia captain Pat Cummins said he was "gutted" to be ruled out just hours before the second Ashes Test against England Thursday for being in close contact with a positive Covid-19 case, with Steve Smith making a dramatic return as captain.

The world's number one Test fast bowler was at an Adelaide restaurant on Wednesday evening when a person at the neighbouring table was identified as positive for coronavirus.

Cummins isolated when he became aware of the situation and has since returned a negative Covid test.

But under South Australia state's strict bio-security rules he must now stay in his hotel room for seven days, meaning Smith captains the side for the first time since a ball-tampering scandal in 2018.

"Gutted to miss this Test but really excited to see (Michael) Neser finally get his chance in the baggy green," Cummins tweeted, referring to his replacement in the bowling attack.

Australian captain Pat Cummins at a press conference on Wednesday
Australian captain Pat Cummins at a press conference on Wednesday AFP / William WEST

"He has done the hard yards and is a seriously skilful player. Super frustrating but Covid has thrown us all some curve balls over the last couple of years. Will be cheering along!"

Australian players are permitted to dine out in small groups under Covid rules in place for the Ashes in Adelaide, but should steer clear of densely populated places.

Cricket Australia chief Nick Hockley said Cummins hadn't breached any of their protocols and should be available for the third Test in Melbourne on December 26.

"We have protocols that flex according to the risk level of any different jurisdiction so we are looking at it every single day," he said.

"I think were pretty comfortable with the protocols for South Australia and we will keep monitoring the situation in Melbourne and in Sydney.

"We don't want to completely lock down the players, we are very conscious of taking a proportionate approach, very conscious of their mental wellbeing," he added.

Steve Smith will captain for the first time since 2018 when he was axed for 12 months and banned from any leadership role for two years over his involvement in attempts to alter the ball illegally with sandpaper
Steve Smith will captain for the first time since 2018 when he was axed for 12 months and banned from any leadership role for two years over his involvement in attempts to alter the ball illegally with sandpaper AFP / Brenton Edwards

Cummins' absence leaves Australia without arguably their best two fast bowlers for a pink ball Test under lights, with Josh Hazlewood also missing with a side injury.

But the situation could have been much worse with fellow quick Mitchell Starc and spin great Nathan Lyon dining in the same restaurant, but at a separate table outdoors.

"SA Health has deemed them casual contacts and they are free to play," Cricket Australia said.

Cummins missing out opened the door again for Smith, who skippered his country until 2018 when he was axed for 12 months and banned from any leadership role for two years over his involvement in attempts to alter the ball illegally with sandpaper during a Test against South Africa in Cape Town.

The sanction expired in March last year and the star batsman had made clear he wanted another chance, with his redemption complete when he was made Cummins' deputy after Tim Paine resigned as captain over a lewd text-message scandal last month.

"I think we're just very fortunate that we've got Steve, he's a very experienced Test captain ready to step in," said Hockley.

"He's obviously served his ban in full. He has, by his own admission, grown a lot as a person.

"The national selection panel and the board felt that he's had demonstrated that he is absolutely fit to hold a leadership position again."

Travis Head, who scored a rollicking century in Australia's nine-wicket win at the first Test in Brisbane after being recalled to the side, took over as vice-captain on his home ground.